Under the 3GPP standards, a NodeB (or an eNB in LTE (i.e. Long Term Evolution)) is the base station module via which mobile devices connect to the core network and which defines a cell of the network. Home base stations or base station modules (HNB) provide short range 3G radiofrequency (RF) coverage, and are sometimes referred to as femto access points (FAP). Where the home base station is operating in accordance with the LTE standards, the HNB is commonly referred to as an HeNB. Some base stations, sometimes referred to as HNB/HeNB, can operate in accordance with both the 3G and LTE standards, and therefore define a dual mode femtocell. The HNB/HeNB base station may also sometimes be referred to as a dual mode femto access point (FAP) or dual FAP.
The HNB or HeNB provides radio coverage (for example, 3G, 4G and/or WiMAX) within the home, small or medium enterprise, shopping malls, etc. and typically connects to the core network via a suitable residential gateway RG and public or corporate broadband (BB) access network (for example via an ADSL link to the Internet). The HNB or HeNB is given an IP address by the local network to which it is connected, and the HNB or HeNB provides this IP address to the mobile telephone operator network so that it can reserve appropriate resources for the HNB or HeNB through the broadband access network.
During operation in a normal operation mode or normal state, the HNB or HeNB enables users of a User Equipment (UE) to communicate with other such users via one of a number of the base stations (eNodeB, eNB) and a core network.
In a connected or active state or mode, a UE is registered with the network and has an RRC (Radio Resource Control) connection with a base station, so that the network can identify which cell the UE belongs to and can transmit data to and receive data from the UE. In LTE, in the active state or mode, the Handover procedure allows UEs to have service continuity while moving within the Intra LTE system (Intra RAT and Inter Frequency) and towards other RATS (Radio Access Technologies).
A UE also has a power conservation or idle state or mode in which, typically, the UE is not transmitting or receiving data, and no context about the UE is stored by the base station. In the idle state, the location of the UE is known only (to the MME (Mobility Management Entity) in 3GPP) at the granularity of a Tracking Area (TA) comprising a cluster or group of base station cells. When in the idle state, a UE selects and reselects cells according to the parameters broadcast by the base station in the BCH (Broadcast Channel), with a frequency given by a Tracking Area Update Timer Value, and the base station is not aware of the cell selections/reselections made by the UE.
During the normal operation mode, the HNB or HeNB usually periodically broadcasts a downlink transmission comprising reference signals and system information, for instance both the Pilot and the Broadcast Channels, to signal its presence to UEs. Each HNB or HeNB also checks if there are any UEs in the idle or connected state in its cell. In order to allow the HNB or HeNB to perform this check, the periodic Tracking Area Update Timer Value may be sent to the HNB or HeNB through the Operations and Management (O&M) system. If no UE responds by performing the Tracking Area Update, the HNB or HeNB can assume that there is no idle state UE present in the cell. The HNB or HeNB may also perform a dummy paging for an idle state UE. If the HNB or HeNB does not receive a paging response from a UE, the HNB or HeNB can assume that there is no idle state UE present in the cell. If no connected or idle state UE is present in the cell for certain duration of time (as determined by an Idle Timer), the HNB or HeNB moves to the energy saving mode, in which it stops all downlink transmissions. However, if an energy saving mode timer expires or if the HNB or HeNB receives a paging or any other message on its S1 interface or TR069 interface (using protocol messages defined in TR 196 Data Model from Broad Band Forum) or if the HNB or HeNB detects an uplink transmission from a UE in a neighbouring cell, then the HNB or HeNB moves back to the normal operation mode, in which it starts downlink transmission again.
In dual mode cells, both the baseband and the radiofrequency processing hardware blocks of the HeNB are independent from the baseband and the radiofrequency processing hardware blocks of the HNB.
However, at least one of the base station modules of a dual mode FAP may need at least one information element from the other base station module. Typical but not limiting operating modes of a dual FAP in which at least one of the base station modules may need at least one information element from the other base station module include NMM information element updating, Handover and Circuit-Switched fallback (CSFB).
NMM Information Element Updating
The base station modules may comprise a Network Monitor Mode (NMM) module which is controlled by an application level software module (sometimes referred to as NMM Controller) and which is configured to operate in a Network Monitor Mode to scan and monitor neighbour cells of RAT such as 2G, 3G, LTE, etc. The scan of the neighbour cells comprises an identification of the neighbour cells and an update of a Neighbour Cell List which includes identification of the identified neighbour cells. The monitoring of the neighbour cells may comprise the measure of at least one of Physical Cell Identifier (PCI), a Primary Scrambling Code (PSC) and a carrier frequency of a neighbour cell.
In a dual FAP, the NMM may be operated by only one of the base station modules, or the two base station modules may not operate the NMM at the same time. Also, in case the base station module operating the NMM is a HeNB, the base station module may identify more neighbour cells than the HNB of the dual FAP, by virtue of the Automatic Neighbour Relation (ANR) procedures.
Therefore the NMM module of the base station module which operated the NMM may need to send the information elements corresponding to the results of NMM, such as the updated PCI, PSC, carrier frequency or Neighbour Cell List, to the other base station module. In some examples, the sending of the information elements might be done via a TR-069 Auto Configuration Server (ACS) over the network.
Circuit-Switched Fallback (CSFB)
The HeNB of the dual FAP relies on a packet-only radio access technology (RAT), and not a circuit-switched (CS) technology which is traditionally used for phone calls. The HeNB may therefore need to perform a Circuit-Switched fallback (CSFB), as defined in 3GPP TS 36.300-v9.6.0, in order to make or receive calls. During the CSFB, the UE changes its RAT from LTE technology to a 2G (GSM) or 3G (UMTS) technology which supports circuit-switched services.
There are several CSFB options for a Circuit Switch fallback to UMTS or GSM. One of the CSFB options is a RRC Connection Release with Redirection with Sys Info information element (IE). In an example, if a HeNB needs to perform a CSFB according to this option, the HeNB needs the information element Sys Info of the HNB which will make or receive the call.
Handover
A Handover (HO) is a procedure which changes the serving cell of a UE in RRC_CONNECTED from a source base station module to a target base station module.
The source base station module makes a decision about initiating a handover based on a measurement report from the UE and Radio Resource Management (RRM) information. When a source base station module decides to perform a handover, the source base station module passes all necessary information elements to the target base station module (e.g. E-RAB attributes and RRC context), using signalling which comprises a handover request message. In return, the target base station module prepares the HO by sending all necessary information elements using signalling which comprises a handover request acknowledge to the source base station module. In a HO, Admission Control (AC) may be performed by the target base station module dependent on the received information from the source base station module, to increase the likelihood of a successful HO, if the resources can be granted by target base station module.
However, a source or target base station module can dynamically reselect and change at least one of a Physical Cell Identifier (PCI), a Primary Scrambling Code (PSC) and a carrier frequency. The other base station module of the dual FAP needs the information elements corresponding to the updated PCI, PSC, and carrier frequency so that the HO can happen and can be successful.
Furthermore, the decision about the grant of the resources by a target base station module may depend on information elements such as target cell parameters which may comprise at least one of a static parameter information element, and a dynamic parameter information element. The static parameter information element may comprise at least one of a maximum number of UE that the target base station module is configured to serve (sometimes referred to as MaxUEsServed), the maximum number of closed subscriber group (CSG) members that the target base station module is configured to serve (sometimes referred to as MaxCSGMembers), the maximum number of non CSG members that the target base station module is configured to serve (sometimes referred to as MaxNonCSGMembers). The dynamic parameter information element may comprise at least one of a current number of CSG UE which are served by the target base station module, a current number of non CSG UE which are served by the target base station module, a congestion status of the target base station module, and a load status of the target base station module.
The exchange of the information elements in the examples above has drawbacks.
In the example of an NMM information element updating or in the example of a handover, an NMM may not be performed by a base station module to monitor immediately the reselection and change of at least one of the PCI, PSC and carrier frequency by the other base station module of the dual FAP. The delayed NMM may result in at least one of the base station not having updated information elements as regards to at least one of the PCI, PSC and carrier frequency of the other base station module of the dual FAP. Furthermore, the TR-069 ACS and the NMM module may not send immediately updated information elements to at least one of the base station module as regards to the NMM results. The delayed sending of the NMM results may result in at least one of the base station not having updated information elements as regards to the Neighbour Cell List, and at least one of the PCI, PSC and carrier frequency of neighbour base station modules.
A subsequent handover might therefore fail or might not occur because of the non updated information elements. Moreover, if a handover is initiated and fails because the information elements were not updated, the signalling from the source and target base station modules was an unnecessary signalling over the communications network which may have been detrimental to other signalling and was unnecessary energy consumption.
In the example of a CSFB, the HNB of the dual FAP sends the needed Sys Info to the HeNB via the core network, using signalling involving RIM (RAN (Radio Access Network) Information Management) procedure, as defined in 3GPP TS 36.300-v 9.6.0. This signalling may result in increased traffic towards the Core Network which might be detrimental to the quality of the other traffic to the Core Network and is energy consuming.